This invention relates to vacuum degassing apparatus and more particularly to a vacuum degassing installation having plural vacuum degassing vessels and which is capable of continuous operation.
One type of vacuum degassing vessel includes one or more downwardly projecting nozzles for drawing metal contained in a ladle upwardly into the vessel for degasification. The lower end of the nozzle is submerged beneath the level of metal in the ladle whereby the metal is drawn at least partway into the vessel as the result of the vacuum therein. In two nozzle vessels, additional metal is conveyed upwardly through a first nozzle by some additional agencies such as a gas lift or an electromagnetic pump. The degassed metal then flows back to the ladle through the second nozzle. Examples of two nozzle vessels are shown in U.S. Pat. Nos. 2,893,860 and 2,994,602.
In single nozzle vessels, metal is moved into and out of the vacuum degassing vessel by cyclically lowering and elevating the same relative to the metal contained with the ladle. Metal is forced into the vessel when there is relative movement of the ladle and vessel toward each other and discharged when separation of the two is increased. An example of a single nozzle vessel is shown in U.S. Pat. No. 2,967,768.
In order to exclude slag which normally covers the molten metal in the ladle, slag shields or breakers are usually affixed to the lower ends of each nozzle between each successive treating cycle. These shields consist of conical shields of about the same composition as the metal being processed. As the lower end of the nozzles are submerged into the metal, the shield prevents entry of slag into the nozzle but soon melts to permit the entry of molten metal into the nozzle.
Additional routine maintenance must also be performed on vacuum degassing vessels. For example, with a typical two-nozzle vessel, slag must be removed from the nozzle and routine nozzle maintenance performed after every five heats requiring about forty-five to one hundred minutes. In addition, the nozzles must be replaced about every sixty heats requiring about one hour and twenty-five minutes. About every one hundred twenty heats, the vessel bottom must be replaced requiring about one hour and twenty-five minutes. Finally, about every 2,500 heats, the upper vessel must also be replaced requiring about six to eight hours. Thus, in an average of twenty shifts, a vessel is available for use only about 80% of the time.
Prior art attempts to increase vessel availability involve mounting a pair of vessels on a turntable with one vessel being mounted in an operative position and a second in a repair position. When it is desired to alternate vessels, the turntable is rotated to move one vessel from the operative to the repair position and the vessel previously in operation is moved to the maintenance position. Such a system is shown in U.S. Pat. No. 3,756,584. This arrangement still does not provide 100% vessel availability because of the time required to disconnect the first vessel from the vacuum system to move the first vessel out of the operative position and the second vessel into its place and then to reconnect the vacuum system to the second vessel. In addition, in such prior art systems, it was not possible to test the vacuum connections of the vessel in the repair position.